The present invention relates to a coating-powder spray gun.
Accordingly the invention relates to a coating-powder spray gun comprising at least one high-voltage electrode electrostatically charging the powder, a powder tube made of an electrically insulating material situated at an input side in the powder spray-gun, a hookup tube made of an electrically insulating material and of which the front end is connected to the rear end of the powder tube and of which the rear end can be plugged into a powder hose, an electrically conducting bush enclosing and thereby sealing the powder tube and which can be grounded to shunt electrical charges.
FIG. 3 of the attached drawings shows a hose/powder-tube hookup of this kind in a known coating-powder spray gun. A bush 6 made of electrically conducting aluminum is slipped onto the rear terminal segment 2 of a powder tube 4 running from the hookup end of a powder spray gun 5 into this gun, and said bush is bonded to the powder tube. The bush is fitted at its front end with an outer thread 8 to allow screwing it into a threaded borehole inside the powder spray gun. A hookup tube 10, or a hookup nipple, is inserted into said bush""s rear terminal segment 12 which projects beyond the rear end of the powder tube 4, said tube 10 or nipple being sealed by an O ring 14 with respect to the bush 6. A powder hose 16 can be plugged onto the rear terminal segment of the hookup tube 10 projecting from the bush 6. The bush 6 can be connected to electrical ground. The bush 6 is electrically conducting and is separated for instance by 300 mm from one or more high-voltage electrodes 18 of the powder spray gun 5 which is sketched here in merely schematic manner. This feature meets the operator""s electrical-safety requirement (operator exposure to arcing and currents), however in extreme conditions there will be danger of the electrical potential breaking down at the high-voltage electrode 18 if metallic powder (coating powder containing metal powder or metal particles) is used for coating objects. As regards a number of different kinds of metallic powders, the metal particles deposit in unwanted manner on the inside of the powder tube 2, of the hookup tube 10 and of the powder hose 16. These deposited metal particle constitute an electrically conducting layer which may shunt the high voltage between the mutually adjoining end faces 15, 17 of the powder tube 4 and of the hookup tube 10 to the bush 6 and hence to ground. This effect is the more pronounced the closer the high-voltage electrode 18 shall be to the bush 6. The powder flow per se causes the high-voltage breakdown, because said flow is also somewhat conductive. The high-voltage electrode(s) 18 is situated near or inside a mouth 20 of a atomizing nozzle which atomizes the coating powder 22 and sprays it onto an object to be coated.
Similar high-voltage coating-powder spray guns are known from the patent documents U.S. Pat. No. 5,022,590 (EP 0 383 031 B1) and U.S. Pat. No. 4,196,465 (DE 28 51 006 C2).
The objective of the present invention is to prevent in simple manner the high-voltage breakdown of the minimum of one high-voltage electrode of the powder spray gun even when the coating powder is a metallic powder.
The invention solves this by means of the features of claim 1.
Accordingly a high-voltage powder spraygun of the invention is characterized in that the front terminal segment of the hookup tube and the rear terminal segment of the powder tube are inserted into each other in axially overlapping and airtight manner so that they constitute between themselves an electrically insulated expanse precluding electric currents between their inside and their outside and in that the bush runs axially as far as or beyond the outer overlap end of the hookup-tube/powder-tube connection to shunt any electric charges that might occur in spite of the said insulated expanse at the outer overlap end between the hookup tube and the powder tube.
The dependent claims disclose further features of the invention.